At first sight, our continuous treatment looks identical to the analysis [1]}, [2]}, [3]}, [4]} of the vWZ problem. There is a caveat, however, which we now demonstrate by computing the total momentum and the energy of the system. The momentum \(\mathcal {P}=\int dx\,\rho (x,t) v(x,t)\)  reduces to
\(\mathcal {P}=\rho _\infty \delta \,\frac{R^2}{t} \int _{-\infty }^1 d\xi \,G(\xi )V(\xi ),\) 
